The present invention relates to a manufacturing technique of semiconductor devices, particularly to a technique effective when applied to improvement of bonding property upon soldering.
Soldering is known as a bonding means of an external terminal to the electrode of a substrate upon packaging of a semiconductor device on the substrate. Instead of a conventional Pbxe2x80x94Sn based solder so far employed mainly, an Sn based solder free of Pb has come to be used recently.
The electrode of a substrate on which a semiconductor device is to be mounted has a structure wherein Nixe2x80x94Au has been adhered to a Cu interconnect by electroplating. On this electrode, the semiconductor device is mounted using a Pbxe2x80x94Sn solder or an Sn solder.
Owing to a recent progress in high density packaging of a semiconductor device, it becomes difficult to adhere Nixe2x80x94Au onto a Cu interconnect by electroplating. Because the smaller a semiconductor device, the greater the limitation on the wiring substrate; and interconnects for supplying power for electroplating which must be connected to each of plural electrodes on the wiring substrate cannot be formed easily by using interconnects on the wiring substrate.
In particular, as the device becomes smaller, the pitch of lands for solder bump electrodes formed over the side opposite to the principal surface of the wiring substrate (opposite-side electrodes) becomes narrow. In addition, when the electrodes are arranged in the array form with plural rows and columns over the opposite side of the wiring substrate, it becomes difficult to dispose interconnects for supplying power for electroplating between these electrodes on the opposite side.
In a multi-chip module (electronic device) having a plurality of semiconductor chips mounted thereon, signal interconnects for connecting a plurality of chips are formed over the principal surface of the wiring substrate so that it becomes difficult to form interconnects for supplying power for electroplating even over the principal surface of the wiring substrate.
As a result, a substrate having electrodes to which Nixe2x80x94Au has been adhered by electroless plating has come to be used.
An Ni film is formed, for example, by electroless plating in a phosphoric acid bath. When the concentration of phosphorous in the Ni film becomes 6 wt. % or less, the quality of the Ni film deteriorates, leading to lowering in its bonding strength to a solder bump. This lowering in the bonding strength of the electrolessly-plated Ni film to the solder bump occurs because of the following reason.
On the Ni film formed by electroless plating, an Au film is formed by substitution of Au for Ni. At this time, a reaction rate (substitution rate) between Au and Ni becomes higher when the concentration of P (phosphorous) contained in the Ni film becomes lower.
When a phosphorous concentration in the Ni film is 6 wt. % or less, a penetration rate of Au shows a marked increase in an Ni crystal boundary region which has many defects and has a low density so that penetration spreads in the longitudinal direction and a penetrated portion in the form of cracks appears in the Ni film.
At the penetrated portion in the form of cracks which has appeared by marked Au penetration into the Ni film, the concentration of Ni shows a relatively large decrease. Upon melt bonding of a solder bump onto the electrode of a wiring substrate, an Snxe2x80x94Ni alloy is not formed at this penetrated portion, thereby causing such a problem as lowering in the bonding strength between the solder bump and Ni film.
It is therefore preferred to incorporate at least 6 wt. %, more preferably 8 wt. % of phosphorous in the Ni film formed by electroless plating in order to improve bonding strength between the solder bump and the Ni film.
When a Pbxe2x80x94Sn solder or an Sn solder is molten on an electrode having an Ni film which has been formed by the above-described electroless plating and is abundant in phosphorous and the solder is bonded to the electrode, however, Ni of the Ni film penetrates into the solder side, thereby forming an Snxe2x80x94Ni alloy. In general, reactivity between phosphorous and Sn is poor so that with the progress of this reaction, phosphorous (P) dispersed in the Ni film is left in the Ni film and moves toward the boundary portion between the Ni film and the reactive layer of the Snxe2x80x94Ni alloy, at which boundary a highly-concentrated P layer is formed.
When this highly-concentrated P layer is formed, peeling occurs between the electrode of the substrate and the solder, resulting in a problem such as marked hindrance of the reliability of the semiconductor device.
The present inventor considered that the above-described highly-concentrated P layer causes peeling of the solder from the electrode.
A technique for imparting a soldered portion with sufficient strength by suppressing the P concentration in the P layer is described, for example, in Japanese Patent Laid-Open No. 2001-60760.
According to the technique described in the above-described gazette, the electrode of a substrate is formed to have a four-layer structure having a copper pattern, an electrolessly-plated high-concentration Nixe2x80x94P film, an electrolessly-plated low-concentration Nixe2x80x94P film and an electrolessly-plated gold film stacked successively in this order one after another.
By forming the electrode of a substrate to have a four-layer structure, it may be possible to suppress the P concentration in the P layer, thereby lowering the P concentration on the surface of the Nixe2x80x94P film. Owing to a low concentration of P in the Ni film to be Au-plated, however, Au penetrates into the Ni film, thereby forming therein a penetrated portion in the form of cracks. This causes such a problem as lowering in the bonding strength between the solder bump and Ni film.
An object of the present invention is therefore to provide a semiconductor device improved in solder bonding property, and a manufacturing method of such a device, and an electronic device also improved in solder bonding property.
Another object of the invention is to provide a semiconductor device of high reliability and a manufacturing method of such a device, and an electronic device also of high reliability.
The above-described and other objects and novel features of the invention will be apparent from the description herein and accompanying drawings.
Of the inventions disclosed by the present application, typical ones will next be summarized.
In one aspect of the invention, there is thus provided a semiconductor device comprising a wiring substrate, a plurality of protruding electrodes formed over a plurality of electrodes of the wiring substrate, and a semiconductor chip disposed over the wiring substrate, wherein the plurality of electrodes each has a first conductor film containing Cu and a second conductor film formed between the first conductor film and the protruding electrode and containing Ni and P; the plurality of protruding electrodes are each made of a solder containing Sn and any one of Ba, Be, Ca and Mg; and the semiconductor chip is electrically connected to the protruding electrode.
In another aspect of the invention, there is also provided an electronic device comprising a wiring substrate, a plurality of first and second electrodes formed over the wiring substrate, and first and second semiconductor devices each having a plurality of electrodes, wherein the plurality of first electrodes each has a first conductor film formed over the wiring substrate and containing Cu and a second conductor film formed over the first conductor film and containing Ni and P; the plurality of second electrodes each has a third conductor film containing Cu, a fourth conductor film formed over the third conductor film and containing Ni and P, and a fifth conductor film formed over the fourth conductor film and containing Au; the plurality of electrodes of the first semiconductor device are electrically connected to the plurality of first electrodes via a solder containing Sn and any one of Ba, Be, Ca and Mg; and the plurality of electrodes of the second semiconductor device are electrically connected to the plurality of second electrodes via a plurality of Au-containing connectors.
In a further aspect of the present invention, there is also provided a manufacturing method of a semiconductor device, which comprises:
(a) preparing a wiring substrate having a plurality of electrodes each made of a first conductor film containing Cu, a second conductor film formed over said first conductor film and containing Ni and P, and a third conductor film formed over the second conductor film and containing Au,
(b) melting a solder containing Sn and any one of Ba, Be, Ca and Mg over the plurality of electrodes of the wiring substrate prepared in the step (a), and
(c) subsequent to said step (b), solidifying the solder over the plurality of electrodes.